1. Field of the Invention
The embodiments of the invention disclosed herein relate generally to the field of diagnostic and therapeutic medical procedures involving the intravenous infusion of fluids such as contrast-enhanced radiographic imaging as an example and, further, to fluid delivery systems employing techniques to correct for capacitance volume effects in fluid-delivery bodies used in fluid delivery systems.
2. Description of Related Art
In many medical diagnostic and therapeutic procedures, a medical practitioner, such as a physician, injects a patient with a fluid. In recent years, a number of injector-actuated syringes and powered injectors for pressurized injection of fluids, such as contrast media (often referred to simply as “contrast”), have been developed for use in procedures such as angiography, computed tomography, ultrasound, and NMR/MRI. In general, these powered injectors are designed to deliver a preset amount of contrast at a preset flow rate.
Angiography is an example of a radiographic imaging procedure wherein a powered injector may be used. Angiography is used in the detection and treatment of abnormalities or restrictions in blood vessels. In an angiographic procedure, a radiographic image of a vascular structure is obtained through the use of a radiographic contrast medium which is injected through a catheter. The vascular structures in fluid connection with the vein or artery in which the contrast is injected are filled with contrast. X-rays passing through the region of interest are absorbed by the contrast, causing a radiographic outline or image of blood vessels containing the contrast. The resulting images can be displayed on, for example, a video monitor and recorded.
In a typical contrast-enhanced radiographic imaging procedure such as angiography, the medical practitioner places a cardiac catheter into a vein or artery. The catheter is connected to either a manual or to an automatic contrast injection mechanism. A typical manual contrast injection mechanism includes a syringe in fluid connection with a catheter connection. The fluid path also includes, for example, a source of contrast, a source of flushing fluid, typically saline, and a pressure transducer to measure patient blood pressure. In a typical system, the source of contrast is connected to the fluid path via a valve, for example, a three-way stopcock. The source of saline and the pressure transducer may also be connected to the fluid path via additional valves, again such as stopcocks. The operator of the manual system controls the syringe and each of the valves to draw saline or contrast into the syringe and to inject the contrast or saline into the patient through the catheter connection.
Automatic contrast injection mechanisms typically include a syringe connected to a powered injector having, for example, a powered linear actuator. Typically, an operator enters settings into an electronic control system of the powered injector for a fixed volume of contrast and a fixed rate of injection. In many systems, there is no interactive control between the operator and the powered injector, except to start or stop the injection. A change in flow rate in such systems occurs by stopping the machine and resetting the injection parameters. Automation of contrast-enhanced imaging procedures using powered injectors is discussed, for example, in U.S. Pat. Nos. 5,460,609 to O'Donnell and 5,573,515 and 5,800,397 both to Wilson et al.
U.S. Pat. No. 5,800,397, for example, discloses an angiographic injector system having both high pressure and low pressure systems. The high pressure system includes a motor-driven syringe injector pump to deliver radiographic contrast material under high pressure to a catheter. The low pressure system includes, among other things, a pressure transducer to measure blood pressure and a pump to deliver a saline solution to the patient as well as to aspirate waste fluid. A manifold is connected to the syringe pump, the low pressure system, and the patient catheter. A flow valve associated with the manifold is normally maintained in a first state connecting the low pressure system to the catheter through the manifold, and disconnecting the high pressure system from the catheter and the low pressure system. When pressure from the syringe pump reaches a predetermined and set level, the valve switches to a second state connecting the high pressure system/syringe pump to the catheter, while disconnecting the low pressure system from the catheter and from the high pressure system. In this manner, the pressure transducer is protected from high pressures.
A feature disclosed in the Wilson et al. patents relate to synchronizing injection of radiographic contrast material with coronary blood flow and, thus, injecting the contrast material in pulses according to the cardiac cycle. However, it is known from this patent that inertial forces of moving contrast material and expansion of the containers and tubing associated with the system and used to conduct the contrast material to the patient via the catheter can cause a phase lag between movement of the syringe plunger within the injector syringe and movement of contrast material out of the catheter and into the patient. To adjust to the phase lag between syringe plunger movement and contrast injection into the patient, a variable time offset may be entered through a control panel such that the timing of the cardiac cycle can be offset by a selected time. Since the magnitude of the phase lag may be dependent on the frequency of heart rate, an algorithm within a computer associated with the control panel continuously and automatically adjusts the magnitude of the time offset based on the instantaneous heart rate during the injection of contrast material.
Another attempt to correct for “elasticity” errors introduced in a fluid delivery systems used to deliver contrast agent to a patient is known from United States Patent Application Publication No. 2006/0079843 to Brooks et al. This published application discloses a dual head injector that utilizes V-tubing in which the fluid paths for contrast agent and saline remain separate until substantially at the patient. By utilizing this type of V-tubing, the elasticity of the fluid delivery components (e.g., syringe, tubing, etc.) can be accommodated and there is reduced lag time in administration of a desired fluid to a patient. In a disclosed embodiment, two different fluid tubes are coupled via the V-tubing with a dual head injector and which are joined at one fluid entry point substantially at the patient. Thus, two fluid tubes merge together between the syringes of the dual head injector and the patient using the V-tubing.
The Brooks et al. publication further discloses that a Y-tubing arrangement has also been used to merge the flow paths of two syringes in a dual head injector, wherein the separate tubes merge relatively near the syringes so that a single fluid tube exists for the majority of the tubing. However, in this arrangement, the inherent elasticity of the syringes allows back flow from the “driven” syringe to the “non-driven” syringe during a pressure injection. Unless precautions are taken with such common tubing merging arrangements, the contents of the driven syringe may be pushed into the un-driven syringe and contaminate the contents of this syringe. One known solution to this backflow problem is to use check valves in the branch conduits of the Y-tubing fluid path. Additionally, Y-tubing introduces lag time between the supplying of the two different fluids. In particular, the entire contents of the Y-tubing shared portion must be flushed of one fluid before a second fluid can be delivered to the patient.
Moreover, it is known that in typical power injector systems there is inherent elasticity due to compression of the syringe plunger and the expansion of the syringe barrel. The shape and size of the syringe plunger affects the amount of elasticity present as well. The foregoing Brooks et al. publication discloses that the syringe plunger of the un-driven syringe in the dual head injector may be driven to a sufficient displacement to prevent the movement of fluid into the tubing associated with the un-driven syringe due to elasticity. The amount of displacement is a function of pressure present in the driven syringe and the type of syringes in use in the dual head injector. Closed-loop, open loop, and a combined open/closed loop approaches are disclosed in this publication for controlling the displacement movement of the syringe plunger in the un-driven syringe. A closed-loop approach to controlling displacement movement of the syringe plunger in the un-driven syringe entails measure of pressure and/or fluid flow in the driven syringe which is then used to perform closed-loop control of the injector ram associated with the un-driven syringe plunger to prevent back flow into the un-driven syringe due to elasticity. In an open-loop approach, measured values of typical elasticity and pressure in the driven syringe may be used to drive an appropriate displacement movement of the syringe plunger in the un-driven syringe. In a combined open/closed loop approach, the initial displacement applied to the un-driven syringe plunger upon initiation of an injection may be obtained from measured typical values, after which a closed-loop control may be initiated to maintain an equilibrated pressure between the driven and un-driven syringes and/or zero flow rate from the un-driven syringe.
In the foregoing contrast fluid delivery systems, indirect attempts are made to correct for “elasticity” errors that are known to occur when the systems are under pressure, but the proposed solutions are directed to specific/limited applications. The Wilson et al. patents disclose a simple time delay or variable time offset to adjust for a phase lag between syringe plunger movement and contrast injection into the patient, such that the timing of the cardiac cycle can be offset by a selected time. Brooks et al. address the limited problem of syringe plunger compression and syringe barrel expansion in a dual head injector system by operating the un-driven syringe thereof in a manner to prevent the back flow therein from the driven syringe. However, these approaches do not address capacitance or compliance volume errors in a comprehensive manner and, as a result, fail to address the under-delivery or over-delivery of contrast to the patient resulting from compliance or capacitance volume in the system “fluid path” components. As a result, less than optimal injection boluses may result and/or operation of the fluid delivery system can result in relatively large amounts of wasted contrast media.